class Solution {
    bool vis[20][20];
    int dx[4] = { 0, 0, 1, -1 };
    int dy[4] = { 1, -1, 0, 0 };
    int m, n, ret, count;
public:
    int uniquePathsIII(vector<vector<int>>& grid) {
        int x, y;
        m = grid.size(), n = grid[0].size();
        for (int i = 0; i < m; i++)
            for (int j = 0; j < n; j++)
            {
                if (grid[i][j] == 0)
                    count++;
                else if (grid[i][j] == -1)
                    vis[i][j] = true;
                else if (grid[i][j] == 1)
                    x = i, y = j;
            }
        dfs(grid, x, y, 0);
        return ret;
    }

    void dfs(vector<vector<int>>& grid, int i, int j, int pos)
    {
        for (int k = 0; k < 4; k++)
        {
            int x = i + dx[k], y = j + dy[k];
            if (x >= 0 && x < m && y >= 0 && y < n && !vis[x][y])
            {
                if (grid[x][y] == 0)
                {
                    vis[x][y] = true;
                    dfs(grid, x, y, pos + 1);
                    vis[x][y] = false;
                }
                else if (grid[x][y] == 2)
                {
                    if (count == pos)
                    {
                        ret++;
                        return;
                    }
                }
            }
        }
    }
};